Heat Pump Guide
How Heat Pumps Work
Heat pumps provide both heating and cooling by moving heat rather than creating it directly. In summer, they work much like a central air conditioner by removing heat from the home and releasing it outdoors. In winter, the process reverses and the system pulls heat from outdoor air and moves it inside. For North Alabama homeowners, understanding how a heat pump works can make it easier to recognize normal operation, understand auxiliary heat and defrost cycles, compare efficiency ratings, and know when the system may need service.
Quick Answer
A heat pump uses refrigerant, indoor and outdoor coils, a compressor, fans, and a reversing valve to move heat in either direction. During cooling, it moves heat from inside the home to the outdoors. During heating, it reverses the refrigerant cycle and moves heat from outdoor air into the home. Because it transfers heat instead of generating all of its heat directly, a heat pump can be an efficient year-round HVAC option for many North Alabama homes.
What Is a Heat Pump?
A heat pump is an HVAC system that can provide both heating and cooling using the refrigeration cycle. The same basic components that allow an air conditioner to remove heat from a home can also be used in reverse to bring heat into the home during winter.
Unlike a furnace, which creates heat through combustion or electric resistance, a heat pump primarily moves existing heat from one place to another.
That is the central idea behind heat-pump operation: the system transfers heat instead of generating all of it directly.
Unlike a furnace, which creates heat through combustion or electric resistance, a heat pump primarily moves existing heat from one place to another.
That is the central idea behind heat-pump operation: the system transfers heat instead of generating all of it directly.
Main Parts of a Heat Pump System
1
Compressor
The compressor circulates refrigerant through the system and raises the refrigerant's pressure and temperature. It is one of the major components that allows heat to be moved between the indoor and outdoor coils.
Depending on the equipment, the compressor may operate at one speed, in stages, or at variable capacity.
Depending on the equipment, the compressor may operate at one speed, in stages, or at variable capacity.
2
Indoor Coil
The indoor coil transfers heat between the refrigerant and the air inside the home. During cooling, it absorbs heat from indoor air. During heating, it releases heat into the indoor air stream.
The blower then moves conditioned air through the duct system.
The blower then moves conditioned air through the duct system.
3
Outdoor Coil
The outdoor coil exchanges heat with outside air. During summer, it releases heat removed from the home. During winter, it absorbs heat from outdoor air so that the refrigerant can carry that heat indoors.
4
Reversing Valve
The reversing valve is what allows a heat pump to switch between heating and cooling. It changes the direction of refrigerant flow so that the indoor and outdoor coils effectively trade jobs.
Without this component, the system would operate only like a conventional air conditioner.
Without this component, the system would operate only like a conventional air conditioner.
5
Expansion Device
The expansion device controls refrigerant flow and reduces pressure before the refrigerant enters the coil where it will absorb heat.
That pressure change is part of what allows the refrigerant to move heat efficiently through the refrigeration cycle.
That pressure change is part of what allows the refrigerant to move heat efficiently through the refrigeration cycle.
6
Blower and Fans
The indoor blower moves conditioned air through the home, while the outdoor fan moves air across the outdoor coil.
Proper airflow is essential for heat transfer, comfort, efficiency, and system performance in both heating and cooling modes.
Proper airflow is essential for heat transfer, comfort, efficiency, and system performance in both heating and cooling modes.
7
Refrigerant
Refrigerant circulates between the indoor and outdoor equipment and changes pressure and temperature as it moves through the system.
Those changes allow the refrigerant to absorb heat in one location and release it in another.
Those changes allow the refrigerant to absorb heat in one location and release it in another.
8
Thermostat and Controls
The thermostat tells the system whether heating or cooling is needed. Modern heat pumps may also use sensors and control boards to manage staging, variable capacity, auxiliary heat, defrost operation, and other system functions.
How a Heat Pump Works at a Glance
- Cooling mode: heat moves from inside the home to the outdoors.
- Heating mode: heat moves from outdoor air into the home.
- Reversing valve: changes refrigerant flow between heating and cooling.
- Compressor: circulates and pressurizes refrigerant.
- Indoor and outdoor coils: absorb and release heat.
- Blower and fans: move air across the coils.
- Auxiliary heat: can provide additional heating when needed.
- Defrost mode: removes frost from the outdoor coil during winter operation.
Important: A heat pump does not have to make outdoor air feel warm before it can heat the home. Even cool outdoor air contains heat energy that the system can absorb and move indoors.
How Does a Heat Pump Cool a Home?
In cooling mode, a heat pump operates much like a central air conditioner.
The indoor coil absorbs heat from the home's air. The refrigerant carries that heat to the outdoor unit, where the outdoor coil releases it into the outside air. The blower circulates the cooled and dehumidified air through the home.
This is why a heat pump and central AC can provide very similar summer cooling performance when they are properly sized and installed.
The indoor coil absorbs heat from the home's air. The refrigerant carries that heat to the outdoor unit, where the outdoor coil releases it into the outside air. The blower circulates the cooled and dehumidified air through the home.
This is why a heat pump and central AC can provide very similar summer cooling performance when they are properly sized and installed.
How Does a Heat Pump Heat a Home?
In heating mode, the refrigerant cycle reverses. The outdoor coil absorbs heat from outside air, even when the weather is cool. The compressor raises the refrigerant temperature, and the indoor coil releases that heat into the home's air stream.
The blower then distributes the warmed air through the duct system.
Because heat pumps transfer heat instead of creating all of it through resistance heating, they can provide efficient heating under many conditions.
The blower then distributes the warmed air through the duct system.
Because heat pumps transfer heat instead of creating all of it through resistance heating, they can provide efficient heating under many conditions.
How Can a Heat Pump Pull Heat From Cold Air?
Cold air still contains thermal energy. A heat pump uses refrigerant that can absorb heat at low outdoor temperatures, then the compressor increases the refrigerant pressure and temperature so that heat can be released indoors.
As outdoor temperatures fall, there is less heat available and the system may need to work harder. Heating capacity and efficiency can also decline depending on the equipment.
As outdoor temperatures fall, there is less heat available and the system may need to work harder. Heating capacity and efficiency can also decline depending on the equipment.
What Does the Reversing Valve Do?
The reversing valve changes the direction of refrigerant flow when the system switches between heating and cooling.
In cooling mode, the indoor coil acts as the evaporator and the outdoor coil acts as the condenser. In heating mode, those functions reverse.
This is the key component that makes a heat pump different from a conventional cooling-only air conditioner.
In cooling mode, the indoor coil acts as the evaporator and the outdoor coil acts as the condenser. In heating mode, those functions reverse.
This is the key component that makes a heat pump different from a conventional cooling-only air conditioner.
What Is Auxiliary Heat?
Auxiliary heat is a supplemental heating source that can assist the heat pump when additional capacity is needed.
In many all-electric systems, auxiliary heat is provided by electric resistance heat strips inside the air handler. It may operate during colder weather, large thermostat changes, or certain defrost cycles.
Some auxiliary-heat operation is normal, but excessive use can increase energy consumption. Our High Energy Bills guide explains how auxiliary heat can affect winter energy use.
In many all-electric systems, auxiliary heat is provided by electric resistance heat strips inside the air handler. It may operate during colder weather, large thermostat changes, or certain defrost cycles.
Some auxiliary-heat operation is normal, but excessive use can increase energy consumption. Our High Energy Bills guide explains how auxiliary heat can affect winter energy use.
What Is Emergency Heat?
Emergency Heat is a thermostat setting that typically disables normal heat-pump heating and relies on the backup heating source instead.
It is generally intended for situations where the heat-pump portion of the system cannot operate properly, not as a routine setting for cold weather.
If the system only heats correctly in Emergency Heat, the heat-pump side of the system may need diagnosis.
It is generally intended for situations where the heat-pump portion of the system cannot operate properly, not as a routine setting for cold weather.
If the system only heats correctly in Emergency Heat, the heat-pump side of the system may need diagnosis.
What Is Heat Pump Defrost Mode?
During cold, damp weather, moisture can freeze on the outdoor coil. A light coating of frost can be normal during winter operation.
To remove that frost, the heat pump periodically enters defrost mode. The system temporarily reverses operation so that heat can warm the outdoor coil and melt the frost.
Auxiliary heat may operate during this process to help maintain indoor comfort. Steam or water around the outdoor unit during defrost can be normal.
To remove that frost, the heat pump periodically enters defrost mode. The system temporarily reverses operation so that heat can warm the outdoor coil and melt the frost.
Auxiliary heat may operate during this process to help maintain indoor comfort. Steam or water around the outdoor unit during defrost can be normal.
Is Frost on a Heat Pump Normal?
A light frost that clears during normal defrost operation can be normal. Heavy ice that continues building, does not clear, or significantly reduces heating performance is not.
Defrost-control problems, airflow issues, refrigerant problems, sensors, or other system faults may be involved.
If heating performance is poor, see our Heat Pump Not Heating guide for common causes.
Defrost-control problems, airflow issues, refrigerant problems, sensors, or other system faults may be involved.
If heating performance is poor, see our Heat Pump Not Heating guide for common causes.
Why Does Heat Pump Air Feel Cooler Than Furnace Air?
Heat pumps usually deliver lower-temperature supply air than gas furnaces. The air can still be warm enough to heat the home even though it may not feel especially hot at the vent.
Heat pumps also tend to operate for longer periods, providing steadier heating rather than short bursts of very hot air.
This operating style is normal when the system is maintaining the thermostat setting properly.
Heat pumps also tend to operate for longer periods, providing steadier heating rather than short bursts of very hot air.
This operating style is normal when the system is maintaining the thermostat setting properly.
Do Heat Pumps Run Longer Than Furnaces?
Often, yes. Heat pumps are designed to provide steady heating and may run for longer cycles, especially during colder weather.
Long run time by itself does not necessarily indicate a problem. The concern is when the system runs continuously and still cannot maintain temperature, relies heavily on auxiliary heat, or shows another sign of declining performance.
Long run time by itself does not necessarily indicate a problem. The concern is when the system runs continuously and still cannot maintain temperature, relies heavily on auxiliary heat, or shows another sign of declining performance.
How Efficient Are Heat Pumps?
Heat pumps can be highly efficient because they move heat rather than converting all of their electricity directly into heat.
Cooling efficiency is commonly compared using SEER2 and EER2, while seasonal heating efficiency is commonly compared using HSPF2 and other performance data.
Our Understanding HVAC Efficiency Ratings guide explains these measurements and how they relate to real-world performance.
Cooling efficiency is commonly compared using SEER2 and EER2, while seasonal heating efficiency is commonly compared using HSPF2 and other performance data.
Our Understanding HVAC Efficiency Ratings guide explains these measurements and how they relate to real-world performance.
Are Heat Pumps Good for North Alabama?
They can be. North Alabama's relatively moderate winters and long cooling season make heat pumps a practical year-round option for many homes.
The system still needs to be properly sized for both heating and cooling, and the backup-heat strategy should match the home's heating load, electrical service, and comfort goals.
Summer humidity control, ductwork, airflow, and installation quality remain important just as they would with a conventional central AC system.
The system still needs to be properly sized for both heating and cooling, and the backup-heat strategy should match the home's heating load, electrical service, and comfort goals.
Summer humidity control, ductwork, airflow, and installation quality remain important just as they would with a conventional central AC system.
How Is a Heat Pump Different From Central Air?
During cooling mode, the two systems operate very similarly. The major difference is that a heat pump can reverse the refrigeration cycle and provide heating as well.
A conventional central air conditioner requires a separate heating source such as a furnace.
See our Heat Pump vs. Central Air guide for a complete comparison.
A conventional central air conditioner requires a separate heating source such as a furnace.
See our Heat Pump vs. Central Air guide for a complete comparison.
How Is a Heat Pump Different From a Furnace?
A heat pump transfers heat using refrigerant, while a furnace creates heat through combustion or electric resistance.
Heat pumps can usually provide both heating and cooling, while furnaces provide heating only and are typically paired with an air conditioner.
Our Heat Pump vs. Furnace guide explains the differences in efficiency, comfort, operating cost, and cold-weather performance.
Heat pumps can usually provide both heating and cooling, while furnaces provide heating only and are typically paired with an air conditioner.
Our Heat Pump vs. Furnace guide explains the differences in efficiency, comfort, operating cost, and cold-weather performance.
What Can Cause a Heat Pump to Stop Heating Properly?
Poor heating can result from thermostat problems, dirty filters, weak airflow, outdoor-unit problems, refrigerant issues, defrost-control faults, electrical components, compressor problems, or failed auxiliary heat.
A heat pump that is not heating properly does not automatically need replacement. Many causes are repairable once properly diagnosed.
A heat pump that is not heating properly does not automatically need replacement. Many causes are repairable once properly diagnosed.
How Often Should a Heat Pump Be Serviced?
Because heat pumps operate during both heating and cooling seasons, they are commonly serviced twice per year—once before summer and once before winter.
Routine service may include checking coils, airflow, electrical components, refrigerant performance, drainage, thermostat operation, defrost controls, auxiliary heat, and overall system performance.
See our How Often Should HVAC Be Serviced? guide for maintenance timing.
Routine service may include checking coils, airflow, electrical components, refrigerant performance, drainage, thermostat operation, defrost controls, auxiliary heat, and overall system performance.
See our How Often Should HVAC Be Serviced? guide for maintenance timing.
How Much Does a Heat Pump Cost to Install?
Installation cost depends on system type, capacity, efficiency, indoor equipment, backup heat, ductwork, electrical requirements, controls, refrigerant lines, and installation conditions.
See our Heat Pump Installation Cost guide for a detailed breakdown of the factors that affect project pricing.
See our Heat Pump Installation Cost guide for a detailed breakdown of the factors that affect project pricing.
What Should North Alabama Homeowners Know About Heat Pump Operation?
Heat pumps often operate differently from furnaces, especially during winter. Longer heating cycles, lower-temperature supply air, occasional defrost cycles, and some auxiliary-heat use can all be normal.
What matters most is whether the system can maintain indoor comfort efficiently and reliably. Weak airflow, persistent ice, unusually frequent auxiliary heat, high energy use, short cycling, or inability to reach the thermostat setting are stronger signs that the system needs attention.
What matters most is whether the system can maintain indoor comfort efficiently and reliably. Weak airflow, persistent ice, unusually frequent auxiliary heat, high energy use, short cycling, or inability to reach the thermostat setting are stronger signs that the system needs attention.
Heat Pump Operation Checklist
- Confirm the thermostat is set to the correct heating or cooling mode.
- Keep filters clean and airflow unrestricted.
- Keep the outdoor unit clear of leaves and debris.
- Expect longer heating cycles during cold weather.
- Understand that heat-pump supply air may feel less hot than furnace air.
- Watch for normal defrost operation in cold, damp weather.
- Monitor excessive ice that does not clear.
- Pay attention to unusually frequent auxiliary-heat operation.
- Watch for unexpected changes in energy use.
- Schedule maintenance before peak heating and cooling seasons.
Frequently Asked Questions
How does a heat pump heat a house?
A heat pump absorbs heat from outdoor air through the outdoor coil, uses refrigerant and the compressor to raise the temperature, and releases that heat indoors through the indoor coil.
How does a heat pump cool a house?
In cooling mode, a heat pump works like a central air conditioner by absorbing heat from indoor air and releasing it outdoors through the refrigeration cycle.
Can a heat pump work when it is cold outside?
Yes. Outdoor air still contains heat even when temperatures are cold. Heat-pump capacity and efficiency can decline as temperatures fall, so auxiliary or backup heat may assist during colder conditions.
Why does my heat pump go into defrost mode?
Defrost mode removes frost that can form on the outdoor coil during cold, damp weather. The system temporarily reverses operation to warm the outdoor coil and melt the frost.
Are heat pumps good for North Alabama?
Yes. North Alabama's relatively moderate winters and long cooling season make heat pumps a practical heating and cooling option for many homes when the system is properly sized and installed.
In This Guide
- Heat pump basics
- Main components
- Heating operation
- Cooling operation
- Auxiliary heat
- Defrost mode
- North Alabama operation
- Heat pump FAQs
Key Components
- Compressor
- Indoor coil
- Outdoor coil
- Reversing valve
- Expansion device
- Refrigerant
Serving North Alabama
3:16 HVAC provides heat pump installation, repair, maintenance, replacement planning, system sizing, efficiency evaluation, and HVAC service throughout Athens, Madison, Huntsville, Harvest, Hazel Green, Meridianville, East Limestone, Elkmont, and surrounding North Alabama communities.
